The electrophotographic development method is a method of developing by adhering toner particles in a developer to electrostatic latent images formed on a photoreceptor. Developers used in this method are divided into two-component developers composed of toner particles and carrier particles, and one-component developers using toner particles alone.
The development method using the two-component developers composed of toner particles and carrier particles, among these developers, employed the cascade method in past, but predominantly employs the magnetic brush method using a magnet roll at present.
In two-component developers, carrier particles are a carrier material which imparts a desired charge to toner particles while they are mixed with the toner particles in a development box filled with a developer, and transports the charged toner particles to the surface of a photoreceptor to form toner images on the photoreceptor. The carrier particles remaining on a development roll holding a magnet are again returned into the development box, mixed and stirred with fresh toner particles, and used repeatedly in a certain period.
Two-component developers comprise, opposed to one-component developers, carrier particles with functions of charging toner particles by mixing and stirring both types of particles and transporting them, and can be designed more controllably. Therefore, two-component developers are suitable for full-color development devices requiring high-quality images, high-speed printing machines requiring reliability and durability of image sustention, and the like.
Two-component developers used in such a way requires that image characteristics such as the image density, fogging in image, white spots, gradation and resolution exhibit prescribed values from the initial period, and further, these characteristics do not vary during continuous printing period and be stably maintained. For stably maintaining these characteristics, the characteristics of carrier particles contained in two-component developers are required to be stable.
As carrier particles forming two-component developers, iron-powder carriers such as iron powders covered on their surface with an oxide film and iron powders coated on their surface with a resin are conventionally used. Since these iron-powder carriers have a high magnetization and a high conductivity, they have an advantage of easily providing well reproduced images on solid parts.
However, since such iron powder carriers have a high true specific gravity of about 7.8 and too high a magnetization, stirring and mixing with toner particles in a development box becomes liable to generate the fusion of toner constituents to the iron powder carrier surface, so-called toner spent. Such generation of toner spent reduces the available carrier surface area, and is liable to decrease the tribochargeability with toner particles.
The resin-coated iron powder carrier sometimes generates the charge leak due to exfoliation of the surface resin by stresses during endurance and exposure of the core material (iron powder), which has a high conductivity and a low dielectric breakdown voltage. Such charge leak breaks electrostatic latent images formed on a photoreceptor, and generates brush-marks and the like on solid parts, hardly obtaining uniform images. From these reasons, the iron powder carriers such as the oxide-filmed iron powder and resin-coated iron powder come not to be used at present.
In recent years, ferrites, which have a low true specific gravity of about 5.0 and also a low magnetization, are used as carriers in place of the iron powder carriers, and further resin-coated ferrite carriers, in which ferrites are coated on their surface with a resin, are often used, whereby the developer life has been remarkably elongated.
A production method of such a ferrite carrier commonly involves mixing ferrite carrier raw materials in prescribed amounts, calcining, milling, granulating, and thereafter sintering, and, depending on the situation, the calcination is sometimes omitted.
However, such a production method of ferrite carriers has various problems. Specifically, since the sintering process to generate the magnetization by the ferritization reaction commonly uses a tunnel kiln, and sinters raw materials filled in a sagger, the shape is liable to become irregular due to the mutual effect between the particles, especially remarkable in ferrite particles of smaller size, and after the sintering, the particles form blocks, and generate cracks and chips when they are disintegrated, which are incorporated as irregular particles. Besides, in the case of producing ferrite particles of small size, well-shaped particles cannot be made without enhanced milling. Further, since the sintering time necessitates about 12 h including the temperature-rising time, maximum temperature-holding time and temperature-falling time, and blocks formed of particles must be disintegrated after the sintering, the production method has a problem of not having the favorable production stability.
A carrier core material produced by such a sintering method has not only cracked and chipped particles, but many irregular particles, which are deformed particles, so even if a resin coat is formed, a uniform coating is difficult to form. The resin coat is thicker in recessed parts of the particle surface, and thinner in protruded parts thereof. In the parts having a thinner resin coat, the carrier core material is earlier exposed by stress, and the leak phenomenon and widening of the charge quantity distribution are caused, thereby having a difficulty in stabilizing high-quality images in a long period.
For achieving prevention of cracking and chipping, and reduction of irregular particles, prevention of aggregation between particles at the time of sintering is needed. For that, if the sintering is performed in a comparatively low sintering temperature, the stress at disintegration after the sintering becomes low, allowing reduction of cracked and chipped particles, and irregular particles, etc.
However, this case provides a porous particle surface property, a worsened charge-rising due to infiltration of a resin, etc. and much resin of needlessly infiltrated parts, and is economically inferior and unfavorable in both quality and cost.
For solving these problems, a new production method of a ferrite carrier is proposed. For example, Patent Document (Japanese Patent Laid-Open No. 62-50839) describes a production method of a ferrite carrier in which a formulation composed of metal oxides formulated as raw materials for forming a ferrite is passed through a high-temperature flame atmosphere, and is ferritized instantaneously thereby.
However, this production method is performed with the ratio of oxygen amount/combustible gas of not more than 3, so the sintering is difficult depending on the type of ferrite raw materials used. Further, it is not suitable for production of ferrites responding to smaller-sized particles in recent years, e.g. small-sized ferrites of about 20 to 50 μm, and cannot provide spherical uniform ferrite particles.
Patent Document 2 (Japanese Patent Laid-Open No. 3-233464) describes melting carrier raw materials by the direct current plasma method, high-frequency plasma method or hybrid plasma method as a production method of a carrier for an electrophotographic developer.
However, since this method uses an expensive gas such as argon or helium, it is economically very disadvantageous and is not practical.
[Patent Document 1] Japanese Patent Laid-Open No. 62-50839
[Patent Document 2] Japanese Patent Laid-Open No. 3-2233464
As described above, a production method which is excellent in the economical stability and productivity of a spherical resin-coated ferrite carrier for an electrophotographic developer which can maintain a stable resistance and chargeability, and is excellent in the fluidity and the charge rising property, has not been found.